Accurate ES readings require consideration of the refraction of the ES signal through the atmosphere and compensating for it. In addition, because the refractivity in atmosphere is not constant throughout the globe, the ES refractivity of the locale where the ES is located must be determined and accounted for. However, because measuring ES refractivity requires additional sensors configured to estimate the refractivity, it is not a practical solution for a mobile ES system to measure the local ES refractivity every time the mobile ES is relocated.